I. Field of the Invention
This invention relates to flow modifiers for sailboat keels and rudders and more particularly to vortex generators for application to sailboat keels and rudders to achieve boundary layer control.
II. Description of the Prior Art
From 1948 through 1950, H. D. Taylor of the United Aircraft Corporation discovered that small mixing devices, now known as vortex generators can be used to increase the efficiency of diverging wall diffusers, air foils, axial flow fans, burner mixing, etc. The generators are simply energy converters in the form of an array of small, closely spaced, low aspect ratio foils or tabs mounted at an angle of attack to the free air stream on a surface having a boundary layer. Each generator accomplishes a conversion of translational energy into rotational energy in the form of a trailing tip vortex. The induced tip vortices reenergize the boundary layer by mixing the higher velocity air in the main stream with the lower velocity air in the boundary layer. This reenergizing of the boundary layer makes it more resistant to separation from a surface with an adverse pressure gradient. Separation would otherwise result in a leveling off of lift and large increases in drag with increasing air foil angles of attack. This is called stall and results in a loss of efficiency commonly measured as lift/drag (L/D) ratio.
Common applications of this principle are on axial flow fan intake shrouds, diverging diffusers, windmill blades, airplane wings and airplane vertical and horizontal stabilizers. The greatest number of applications have been on airplane wings. The vortex generators are placed on the wing surface at an angle of attack to the fore and aft direction of airplane travel. The foil axes of the vortex generators are either in an alternating plus and minus angle of attack orientation relative to the direction of the airplane travel to generate adjacent counter-rotating vortices, or in a parallel array with the foil axes at a constant angle of attack to the fore and aft direction of the airplane to generate co-rotating trailing vortices.
Due to the angle of attack of the vortex generators relative to the forward motion of the airplane, there is a small drag penalty for using vortex generators on aircraft surfaces at angles of attack which are less than the angle at which wing or control surfaces normally stall. This small drag penalty during normal flight is accepted in order to avoid the large losses in lift and much larger increases in drag during transient conditions requiring larger lift coefficients at higher angles of attack, beyond which, without the vortex generators, such surfaces would otherwise experience stall. The use of vortex generators allows the use of a smaller wing on an aircraft. Consequently, the smaller wing may be brought to a higher angle of attack producing higher lift before stall occurs. Thus, the large drag increase that occurs at stall is delayed until higher angles of attack occur. A relatively complete discussion of boundary layer control and vortex generators is found in Chapter VI of Incompressible Aerodynamics, Bryan Thwaites, published 1960, Oxford at the Clarendon Press.
The only known prior art in sailboats or marine vehicles using any type of foil on keels and rudders have been large lifting foils. These have generally been dimensionally large and used to lift the boat vertically out of the water. To develop such lift the foils are mounted with their foil axes at angles of attack to the fore and aft direction of the boat. The flow caused by the boat's forward motion through the water causes these foils to develop an upwards lift on the boat. Such arrangements are seen in U.S. Pat. No. 4,599,964, Kenny et al, and U.S. Pat. No. 4,606,291, Karl-Gunther W. Hoppe.